Re: [Python-ideas] Function to unnest for-statements

May 23, 2008

      On Fri, May 23, 2008 at 4:38 AM, Brandon Mintern <bmintern@gmail.com> wrote:
...
def combinations (*args):
  def combiner (seqs):
      try:
          seq0 = seqs.next()
      except StopIteration:
          yield ()
      else:
          for prefix in combiner(iter(seqs)):
              for x in seq0:
                  yield prefix + (x,)
  return combiner(reversed(args))
I was incorrectly thinking that combiner would be called more than
once on seqs, and thus I used iter(seqs) to avoid making a call on an
exhausted iterator. It turns out that's unnecessary since there is
only one recursive call per call to combiner, so the result is the
somewhat improved:

def combinations (*args):
    def combiner (seqs):
        try:
            seq0 = seqs.next()
        except StopIteration:
            yield ()
        else:
            for prefix in combiner(seqs):
                for x in seq0:
                    yield prefix + (x,)
    return combiner(reversed(args))

I think this is a solution I can be proud of, in terms of correctness
(iterating in the proper order), readability, and efficiency.

Brandon